The present invention relates to DNA molecules coding for proteins from plants having the enzymatic activity of a debranching enzyme (R enzyme). The invention furthermore relates to a process for modifying the branching degree of amylopectin synthesized in plants, and to plants and plant cells in which an amylopectin having a modified branching degree is synthesized due to the expression of an additional debranching enzyme activity or the inhibition of an endogenous debranching enzyme activity, as well as to the starch obtainable from said plant cells and plants.
Starch plays an important role both as storage substance in a variety of plants and as reproductive, commercially useful raw material and is gaining significance. For the industrial application of starch it is necessary that the starch meets the requirements of the manufacturing industry in terms of its structure, form and/or other physico-chemical parameters. For the starch to be useful in as many fields of application as possible it is furthermore necessary that it is obtainable in as many forms as possible.
While the polysaccharide starch is composed of chemically uniform components, the glucose molecules, it is a complex mixture of different molecule forms that exhibit differences as regards their polymerization degree and the presence of branches. One distinguishes the amylose starch, an essentially unbranched polymer of xcex1-1,4 glycosidically linked glucose molecules from the amylopectin starch, a branched polymer, the branches of which are the result of additional xcex1-1,6 glycosidic bonds.
In plants typically used for starch production, such as maize or potato, both starch forms are present in a ratio of about 25 parts of amylose to 75 parts of amylopectin. In addition to amylopectin, maize, for example, exhibits another branched polysaccharide, the so-called phytoglycogen which differs from the amylopectin by a higher branching degree and a differing solubility (see, e.g., Lee et al., Arch. Biochem. Biophys. 143 (1971), 365-374; Pan and Nelson, Plant Physiol. 74 (1984), 324-328). In the present application the term amylopectin is intended to comprise phytoglycogen.
With a view to the uniformity of the basic compound starch for its industrial application starch-producing plants are required that contain, e.g., either only the component amylopectin or only the component amylose. For other applications plants are required that synthesize forms of amylopectin of different degrees of branching.
Such plants can be generated, e.g., by breeding or mutagenesis techniques. It is known of certain plant species, e.g., maize, that mutagenesis can be used to generate varieties producing only amylopectin. For potato, a genotype was generated by chemical mutagenesis for a haploid line that does not produce amylose (Hovenkamp-Hermelink, Theor. Appl. Genet. 75 (1987), 217-221). Haploid lines, however, or the homozygous diploid or tetraploid lines derived thereof are not useful for agricultural purposes. Mutagenesis techniques, however, cannot be applied to the tetraploid lines that are interesting for agriculture since due to the presence of four different genotypes inactivation of all copies of a gene is not technically feasible. Therefore, in the case of potato, one must fall back on other techniques, e.g., the specific genetically engineered modification of plants.
For example it is known from Visser et al. (Mol. Gen. Genet. 225 (1991), 289) and WO 92/11376 that varieties can be generated by anti-sense inhibition of the gene for the starch granule-bound starch synthase in potato that synthesize substantially pure amylopectin starch.
WO 92/14827 discloses DNA sequences coding for a branching enzyme (Q enzyme) that introduces xcex1-1,6 branches into amylopectin starch. With these DNA sequences it should be possible to generate transgenic plants which exhibit a modified amylose/amylopectin ratio of the starch.
In order to furthermore specifically modify the branching degree of starch synthesized in plants by using genetic engineering it is still necessary to identify DNA sequences coding for enzymes that are involved in starch metabolism, particularly in the branching of starch molecules.
In addition to the Q enzymes that are capable of introducing branches into starch molecules plants comprise enzymes that are capable of dissolving branching. These enzymes are referred to as debranching enzymes and are divided into three groups in terms of their substrate specificity:
(a) Pullulanases that use amylopectin as substrate in addition to pullulan can be found in microorganisms such as Klebsiella and in plants. In plants these enzymes are often referred to as R enzymes.
(b) Isoamylases that do not use pullulan but glycogen and amylopectin as substrate can likewise be found in microorganisms and plants. Isoamylases have been described, e.g., for maize (Manners and Rowe, Carbohydr. Res. 9 (1969), 107) and potato (Ishizaki et al., Agric. Biol. Chem. 47 (1983), 771-779).
(c) Amylo-1,6-glucosidases have been described for mammals and yeasts and use limit dextrins as substrates.
Li et al. (Plant Physiol. 98 (1992), 1277-1284) succeeded in detecting in sugar beet only one debranching enzyme of the pullulanase type in addition to five endoamylases and two exoamylases. Having a size of about 100 kD and a pH optimum of 5.5, this enzyme is localized in the chloroplasts. For spinach, too, a debranching enzyme has been described that uses pullulan as substrate. Both the debranching enzyme of spinach and that of sugar beet possess an activity that is lower by a factor of 5 when reacting it with amylopectin as substrate instead of pullulan as substrate (Ludwig et al., Plant Physiol. 74 (1984), 856-861; Li et al., Plant Physiol. 98 (1992), 1277-1284).
The activity of a debranching enzyme was examined by Hobson et al. (J. Chem. Soc. (1951), 1451) for potato which is a starch-storing cultivated plant, that is important from the agricultural point of view. They succeeded in proving that the corresponding enzymexe2x80x94in contrast to the Q enzymexe2x80x94does not possess chain-extending activity but merely hydrolyzes xcex1-1,6 glycosidic bonds. However, it has not been possible so far to characterize the enzyme in more detail.
For potato processes for the purification of the debranching enzyme as well as partial peptide sequences of the purified protein have been proposed (German patent application P 43 27 165.0 and PCT/EP94/026239). In principle, it should be possible to identify DNA molecules coding for the respective proteins by means of known peptide sequences when using degenerate oligonucleotide probes. However, in practice, often the problem arose that the degree of degeneration of the probe is too high or the probes are too short to specifically identify sequences coding for the desired protein.
Despite the knowledge of the proposed peptide sequences of the debranching enzyme of potato researchers so far have not been, able to isolate DNA molecules coding for debranching enzymes of plants by hybridization to degenerate oligonucleotides or by other genetic or immunological approaches such as proposed in German patent application P 43 27 165.0.
For spinach, too, for which the purification of the debranching enzyme has been described by Ludwig et al. (Plant Physiol. 74 (1984), 856-861), researchers have not been able to either determine peptide sequences or to identify DNA molecules coding for said protein.
The problem underlying the present invention is therefore to provide DNA molecules coding for proteins of plants having the enzymatic activity of a debranching enzyme and allowing to generate transgenic plant cells and plants having an increased or reduced activity of a debranching enzyme.
The problem is solved by the provision of the embodiments described in the claims.
The present invention therefore relates to DNA molecules coding for proteins of plants having the biological activity of a debranching enzyme, or a biologically active fragment thereof.
Such a DNA molecule preferably codes for a debranching enzyme of plants comprising the amino acid sequence indicated in Seq ID No. 18 or Seq ID No. 24. More preferably, such a DNA molecule comprises the nucleotide sequence indicated in Seq ID No. 17 or Seq ID No. 23, particularly the coding region thereof.
The subject matter of the invention are also DNA molecules coding for proteins of plants having the biological activity of a debranching enzyme, or biologically active fragments thereof and that hybridize to any of the DNA molecules described above.
The term xe2x80x9chybridizationxe2x80x9d in this context means hybridization under conventional hybridization conditions, preferably under stringent conditions such as described by, e.g, Sambrook et al. (1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). These DNA molecules that hybridize to the DNA molecules according to the present invention in principle can be derived from any plant possessing such DNA molecules. Preferably, they are derived from monocotyledonous or dicotyledonous plants, preferably from useful plants, and most preferably from starch-storing plants.
DNA molecules hybridizing to the DNA molecules of the present invention can be isolated, e.g, from genomic libraries or cDNA libraries of various plants.
Such DNA molecules from plants can be identified and isolated by using the DNA molecules of the present invention or fragments of these DNA molecules or the reverse complements of these molecules, e.g., by hybridization according to standard techniques (see, e.g., Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).
As hybridization probe, e.g., DNA molecules can be used that have exactly or substantially the same DNA sequence indicated in Seq ID No. 17 or Seq ID No. 23 or fragments of said sequence. The DNA fragments used as hybridization probes can also be synthetic DNA fragments obtained by conventional DNA synthesis techniques and the sequence of which is substantially identical to that of the DNA molecules according to the invention. Once genes hybridizing to the DNA molecules of the invention have been identified and isolated it is necessary to determine the sequence and to analyze the properties of the proteins coded for by said sequence.
The term xe2x80x9chybridizing DNA moleculesxe2x80x9d includes fragments, derivatives and allelic variants of the above-described DNA molecules that code for the above-described protein or a biologically active fragment thereof. Fragments are understood to be parts of DNA molecules long enough to code for the described protein or a biologically active fragment thereof. The term xe2x80x9cderivativexe2x80x9d means in this context that the DNA sequences of these molecules differ from the sequences of the above-described DNA molecules in one or more positions and are highly homologous to said DNA sequence. Homology is understood to refer to a sequence identity of at least 40%, particularly an identity of at least 60%, preferably more than 80% and still more preferably more than 90%. The deviations from the DNA molecules described above can be the result of deletion, substitution, insertion, addition or recombination.
Homology furthermore means that the respective DNA sequences or encoded proteins are functionally and/or structurally equivalent. The DNA molecules that are homologous to the DNA molecules described above and that are derivatives of said DNA molecules are regularly variations of said DNA molecules which represent modifications having the same biological function. They may be naturally occurring variations, such as sequences of other plant species, or mutations. These mutations may occur naturally or may be achieved by specific mutagenesis. Furthermore, these variations may be synthetically produced sequences.
The allelic variants may be naturally occurring variants as well as synthetically produced or genetically engineered a variants.
The proteins encoded by the various variants of the DNA molecules of the invention share specific common characteristics, such as enzymatic activity, molecular weight, immunological reactivity, conformation, etc., as well as physical properties, such as electrophoretic mobility, chromatographic behavior, sedimentation coefficients, solubility, spectroscopic properties, stability, pH optimum, temperature optimum, etc.
Enzymatic activity of the debranching enzyme can be detected in a iodine stain test such as described in Example 5. This test is based on the finding that a protein having a starch-modifying activity can be detected by separating protein extracts, e.g., from tubers, in non-denaturing amylopectin-containing polyacrylamide gels (PAAG) and subsequently staining the gel, after incubation in a suitable buffer, with iodine. While unbranched amylose forms a blue complex with iodine, amylopectin results in a reddish-purple color. Amylopectin-containing polyacrylamide gels giving a reddish-purple color when reacted with iodine result in a change of color up to a blue color of the gel at places where the debranching activity is localized, since the branches of the purple-staining amylopectin are digested by the debranching enzyme.
Alternatively, debranching enzyme activity can be detected by the DNSS test (see Ludwig et al., Plant Physiol. 74 (1984), 856-861).
The present invention furthermore relates to DNA molecules the sequences of which differ from the sequences of the above-identified DNA molecules due to degeneracy of the genetic code, and which code for a protein of a plant having the biological activity of a debranching enzyme or for a biologically active fragment thereof.
According to a preferred embodiment the protein coded for by the DNA molecules according to the invention contains at least one of the peptide sequences depicted in Seq ID No. 1 to Seq ID No. 14.
According to another preferred embodiment the debranching enzymes of plants coded for by the DNA molecules of the invention can be isolated from plant protein extracts by fractionated ammonium sulfate precipitation and subsequent affinity chromatography on xcex2-cyclodextrin. Preferably, the DNA molecules of the invention code for proteins that exhibit a molecular weight between 50 and 150 kD in SDS gel electrophoresis, preferably between 70 and 130 kD, and most preferably between 90 and 110 kD.
In principle, the DNA molecules according to the invention can be derived from any plant organism that expresses the proteins described, preferably from taxonomically higher plants, particularly from monocotyledonous or dicotyledonous plants, preferably from plants that synthesize or store starch. Most preferred are, e.g., cereals (such as barley, rye, oat, wheat, etc.), maize, rice, pea, cassava, etc.
According to a preferred embodiment the DNA molecules of the invention are derived from plants of the family Solanaceae or from plants of the family Chenopodiaceae, preferably from Solanum tuberosum or Spinacia oleracea.
The invention furthermore relates to vectors, particularly plasmids, cosmids, viruses, bacteriophages and other vectors conventional in genetic engineering that contain the above-described DNA molecules of the invention.
According to a preferred embodiment the DNA molecules contained in the vectors are linked to regulatory DNA sequences allowing transcription and translation in procaryotic or eucaryotic cells.
According to another embodiment the invention relates to host cells, particularly procaryotic or eucaryotic cells that have been transformed with a DNA molecule or a vector described above, and cells that are derived from such host cells.
Furthermore, the present invention relates to processes for producing a protein of a plant having the biological activity of a debranching enzyme or a biologically active fragment thereof, wherein host cells according to the invention are cultivated under suitable conditions and the protein is obtained from the culture.
Another subject matter of the invention are the proteins obtainable by said process.
The invention furthermore relates to proteins of plants having the biological activity of a debranching enzyme that are coded for by the DNA molecules of the invention, except for the proteins obtained from spinach and potato that have already been described.
By providing the DNA molecules of the invention it is possible to genetically engineer plant cells such that they exhibit an increased or reduced debranching enzyme activity as compared to wild type cells. Such a modified starch is suitable for various purposes.
According to a preferred embodiment the host cells of the invention are transgenic plant cells that exhibit an increased debranching enzyme activity as compared to non-transformed cells due to the presence and expression of an additionally introduced DNA molecule of the invention.
Another subject matter of the invention are transgenic plants containing the transgenic plant cells described above.
The invention furthermore relates to the starch obtainable from the transgenic plant cells or plants. Due to the increased debranching enzyme activity the amylopectin starch synthesized by the transgenic cells or plants has properties differing from those of starch from non-transformed plants. For example, when analyzing the viscosity of aqueous solutions of this starch upon treating they display a maximum viscosity lower than that of starch of non-transformed plants. Preferably the value of the maximum viscosity is reduced by at least 40%, particularly by at least 55% and still more preferably by at least 65% in comparison with the maximum viscosity of starch from wild type plants. Furthermore, the final viscosity of aqueous solutions of the modified starch after cooling is higher than that of wild type starch. Preferably, the final viscosity is at least 10% higher, particularly at least 30% and still more preferably at least 50% higher than that of starch form wild type plants.
Moreover, the stability of gels consisting of the modified starch is higher than that of gels of wild type starch. The force that is required to deform gels of the modified starch is greater by a factor of at least 2.5, particularly of at least 3.5 and still more preferably of at least 5.5 than that required to deform gels of wild type starch. Furthermore, the phosphate content of the modified starch is comparable to that of wild type starch.
Another object of the invention is the use of the described starch for the production of food and industrial products.
Another subject matter of the invention is propagating material of the plants of the invention, such as seeds, fruit, cuttings, tubers, root stocks, etc., with this propagating material containing transgenic plant cells described above.
The present invention furthermore relates to transgenic plant cells in which the activity of the debranching enzyme is reduced due to the inhibition of the transcription or translation of endogenous nucleic acid molecules coding for a debranching enzyme. This is preferably achieved by expressing a DNA molecule of the invention in the respective plant cells in antisense direction. Due to an antisense effect the debranching enzyme activity is reduced. Another possibility of reducing the debranching enzyme activity in plant cells is to express suitable ribozymes that specifically cleave transcripts of the DNA molecules of the invention. The production of such ribozymes using the DNA molecules of the invention is known in the art. Alternatively, the debranching enzyme activity in the plant cells may be reduced by a co-suppression effect.
The invention furthermore relates to transgenic plants containing the transgenic plant cells described above having reduced debranching enzyme activity. Another subject matter of the invention is the modified starch obtainable from the transgenic cells or plants. The amylopectin starch of the transgenic cells and plants exhibits an altered branching degree as compared to the starch of non-transformed plants due to the reduced debranching enzyme activity. Furthermore, the modified starch obtainable from the described transgenic plants may differ in several aspects form starch of wild type plants. For example, when analyzing the viscosity of aqueous solutions of this starch upon heating they display a maximum viscosity that is lower than that of starch from non-transformed plants. Preferably, the value of the maximum viscosity is reduced by at least 35%, particularly by at least 40% and still more preferably by at least 50% in comparison to the maximum viscosity of starch from wild type plants.
Starch granules of the modified starch synthesized by plants with reduced debranching enzyme activity may have a rough, chapped or even frayed surface.
Furthermore, the modified starch is characterized in that gels produced from this starch are more stable than gels of wild type starch. The force that is required to deform gels of the modified starch is greater by a factor of at least 2.3, more preferably by at least 3.8 and still more preferably by at least 6.0 than that required to deform gels of wild type starch.
The phosphate content of the modified starch is preferably higher than that of wild type starch. The increase in phosphate content depends on the degree of reduction of the debranching enzyme activity. Preferably, the phosphate content is at least 15%, more preferably at least 25% and still more preferably at least 60% higher than that of wild type starch.
Still another object of the invention is the use of the described starch for the production of food or industrial products.
The invention also relates to propagating material of the above-described transgenic plants, such as seeds, fruit, cuttings, tubers, root stocks, etc., with the propagating material containing above-described transgenic plant cells.
Transgenic plant cells that due to the additional expression of a debranching enzyme generate an amylopectin starch having an altered branching degree as compared to the amylopectin starch synthesized by wild type plants are obtainable, e.g., by a process comprising the following steps:
(a) Producing an expression cassette comprising the following DNA sequences:
(i) a promoter allowing transcription in plant cells;
(ii) at least one DNA sequence coding for a protein having the enzymatic activity of a debranching enzyme and being fused to the 3xe2x80x2 end of the promoter in sense orientation; and
(iii) optionally a termination signal for the transcription termination and the addition of a poly-A tail to the transcript formed that is coupled to the 3xe2x80x2 end of the coding region; and
(b) transformation of plant cells with the expression cassette produced in step (a).
Transgenic plant cells that due to the reduction of the activity of a debranching enzyme generate an amylopectin starch having an altered branching degree as compared to the amylopectin starch synthesized by wild type plants are obtainable, e.g., by a process comprising the following steps:
(a) Producing an expression cassette comprising the following DNA sequences:
(i) a promoter allowing transcription in plant cells;
(ii) at least one DNA sequence coding for a protein having the enzymatic activity of a debranching enzyme or at least part of such a protein and being fused to the 3xe2x80x2 end of the promoter in anti-sense orientation; and
(iii) optionally a termination signal for the transcription termination and the addition of a poly-A tail to the transcript formed that is coupled to the 3xe2x80x2 end of the DNA sequence defined in (ii); and
(b) transformation of plant cells with the expression cassette produced in step (a).
In principle, any promoter that is functional in plants can be used as the promoter mentioned in (i). The promoter may be homologous or heterologous with respect to the plant species used. A suitable promoter is, e.g., the 35S promoter of the cauliflower mosaic virus (Odell et al., Nature 313 (1985), 810-812) which allows constitutive expression in all tissues of a plant, and the promoter construct described in WO94/01571. However, promoters can be used that lead to expression of subsequent sequences only at a certain point of time determined by external factors (see, e.g., WO93/07279) or in a certain tissue of the plant (see, e.g., Stockhaus et al., EMBO J. 8 (1989), 2245-2251). Preferably promoters are used that are active in the starch-storing organs of the plants to be transformed. These starch-storing organs are, e.g., the maize grains in maize while it is the tubers in potato. For the transformation of potato, the tuber-specific B33 promoter (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) is particularly, but not exclusively, useful. Provided that the DNA sequence mentioned under process step (a)(ii) coding for a protein having the enzymatic activity of a debranching enzyme is linked to the promoter in sense orientation, this DNA sequence can be of native or homologous origin or of foreign or heterologous origin with respect to the plant species to be transformed.
The use of the DNA molecules of the invention is preferred. In principle, the synthesized protein can be localized in any compartment of the plant cell. Debranching enzymes of plants are regularly localized in the plastids and therefore possess a signal sequence for the translocation in these compartments. In the case of the amino acid sequence depicted in Seq ID No. 17 the signal sequence consists of the 64 N-terminal amino acids. In order to achieve localization in another compartment of the cell, the DNA sequence coding for said signal sequence must be removed and the coding region must be linked to DNA sequences allowing localization in the respective compartment. Such sequences are known (see, e.g., Braun et al., EMBO J. 11 (1992), 3219-3227; Wolter et al., Proc. Natl. Acad. Sci. USA 85 (1988), 846-850; Sonnewald et al., Plant J. 1 (1991), 95-106).
Provided that the DNA sequence mentioned under process step (a) (ii) coding for a protein having the enzymatic activity of a debranching enzyme is linked to the promoter in anti-sense orientation, this DNA sequence preferably is of homologous origin with respect to the plant species to be transformed. However, DNA sequences can also be used that exhibit a high degree of homology to endogenous present debranching enzyme genes, particularly homologies over 80%, preferably homologies between 90% and 100% and most preferably homologies over 95%.
Sequences down to a minimum length of 15 bp can be used. An inhibiting effect, however, cannot be excluded even if shorter sequences are used. Preferred are longer sequences between 100 and 500 base pairs; for an efficient anti-sense inhibition sequences with a length of more than 500 base pairs are particularly used. Usually, sequences are used that are shorter than 5000 base pairs, preferably sequences that are shorter than 2500 base pairs.
In the case of the transformation of potato the DNA sequence preferably is the DNA sequence depicted in Seq ID No. 23 or parts thereof that are long enough to produce an anti-sense effect.
Termination signals for the transcription in plant cells have been described and can be freely interchanged. For example, the termination sequence of the octopin synthase gene from Agrobacterium tumefaciens can be used.
The transfer of the expression cassette constructed according to process step (a) to plant cells is preferably brought about by using plasmids, particularly plasmids that allow stable integration of the expression cassette into the plant genome.
In principle, the processes described above can be applied to all plant species. Of interest are both monocotyledonous and dicotyledonous plants. For various monocotyledonous and dicotyledonous plant species transformation techniques have already been described. The processes are preferably applied to useful plants, in particular starch-producing plants, such as cereals (such as maize, wheat, barley, rye, oat), potato, pea, rice, cassava, etc.
For the preparation of the introduction of foreign genes into taxonomically higher plants there is a wide choice of cloning vectors that contain a replication signal for E. coil and a marker gene for the selection of transformed bacterial cells. Examples of such vectors are pBR322, pUC series, M13mp series, pACYC184, etc. The desired sequence can be introduced into the vector at a suitable restriction site. The plasmid obtained is used to transform E. coli cells. Transformed E. coli cells are cultivated in a suitable medium, harvested and lysed. The plasmid is recovered according to standard techniques. As methods for the analysis for the characterization of the obtained plasmid DNA restriction analyses and sequence analyses are generally used. After each manipulation the plasmid DNA can be cleaved and the resulting DNA fragments can be linked to other DNA sequences.
There is a large number of techniques available for the introduction of DNA into a plant host cell. These techniques include the transformation of plant cells with T-DNA using Agrobacterium tumefaciens or Agrobacterium rhizogenes as transformant, the fusion of protoplasts, injection, electroporation of DNA, the introduction of DNA via the biolistic technique and other possible techniques.
In the case of injection and electroporation of DNA into plant cells no specific requirements are made to the plasmids used. Simple plasmids such as pUC derivatives can be used. If, however, one intends to regenerate whole plants from the respectively transformed cells, it is necessary that a selectable marker gene is present.
Depending on the method of introduction of the desired genes into the plant cell further DNA sequences can be necessary. If, e.g., the Ti or Ri plasmid is used to transform the plant cell, at least the right border, often, however, the right and left border of the Ti and Ri plasmid T-DNA must be linked as flanking region to the genes to be introduced.
If Agrobacteria are used for transformation, the DNA to be introduced must be cloned into special plasmids, either into an intermediate vector or into a binary vector. The intermediate vectors can be integrated by homologous recombination into the Ti or Ri plasmid of the Agrobacteria due to sequences that are homologous to sequences in the T-DNA. Said plasmid contains the vir region necessary for the transfer of the T-DNA. Intermediate vectors are not able to replicate in Agrobacteria. The intermediate vector can be transferred to Agrobacterium tumefaciens using a helper plasmid (conjugation). Binary vectors are able to replicate both in E. coli and in Agrobacteria. They contain a selection marker gene and a linker or polylinker flanked by the right and left T-DNA border regions. They can be directly transformed into Agrobacteria (Holsters et al., Mol. Gen. Genet. 163 (1978), 181-187). The plasmids used for transformation of Agrobacteria contain furthermore a selection marker gene allowing selection of transformed bacteria, such as the NPTII gene. The Agrobacterium serving as host cell should contain a plasmid carrying a vir region. The vir region is necessary for the transfer of the T-DNA to the plant cell. Additional T-DNA may be present. The Agrobacterium so transformed is used to transform plant cells.
The use of T-DNA for the transformation of plant cells has been extensively examined and is sufficiently described in EP 120516; Hoekema, In: The Binary Plant Vector System, Offsetdrukkerij Kanters B.V., Alblasserdam, Chapter V; Fraley et al., Crit. Rev. Plant. Sci., 4:1-46 and An et al., EMBO J. 4 (1985), 277-287. Some binary vectors are already commercially available, e.g., pBIN19 (Clontech Laboratories, Inc. USA).
For the transfer of the DNA to the plant cells plant explants can expediently be cocultivated with Agrobacterium tumefaciens or Agrobacterium rhizogenes. From the infected plant material (e.g., pieces of leaves, stem segments, roots but also protoplasts or suspension-cultivated plant cells) whole plants can be regenerated on an appropriate medium which may contain antibiotics or biocides for the selection of transformed cells. The plants thus obtained can be screened for the presence of the introduced DNA.
Once the introduced DNA is integrated into the genome of the plant cell, it generally remains there stably and can also be found in the successors of the originally transformed cell. Normally it contains a selection marker which imparts to the transformed plant cells resistance to a biocide or an antibiotic such as kanamycin, G 418, bleomycin, hygromycin or phosphinotricin etc. The selected marker should therefore allow for the selection of transformed cells over cells lacking the introduced DNA.
The transformed cells grow within the cell as usual (cf., e.g., McCormick et al., Plant Cell Reports 5 (1986), 81-84). These plants can be grown in the usual manner and can be cross-bred with plants possessing the same transformed genetic material or other genetic materials. The resulting hybrid individuals have the corresponding phenotypic properties.
Two or more generations should be cultivated in order to make sure that the phenotypic features are stably retained and inherited. Furthermore, seeds should be harvested in order to make sure that the corresponding phenotype or other characteristics have been retained.
The introduction of an expression cassette constructed according to the processes described above results in the formation of RNA in the transformed plant cells. If the DNA sequence coding for a debranching enzyme is linked with the promoter in sense orientation in the expression cassette, mRNA is synthesized which may serve as template for the synthesis of an additional or new debranching enzyme in the plant cells. As a result, these cells exhibit an increased debranching enzyme activity leading to a change of the branching degree of the amylopectin produced in the cells. By this change a starch becomes available that excels vis-à-vis the naturally occurring starch by a more coordinate spatial structure and a higher uniformity. This has inter alia favorable effects on the film-forming properties of the starch.
If the DNA sequence coding for a debranching enzyme is linked with the promoter in anti-sense orientation, an anti-sense RNA is synthesized in transgenic plant cells that inhibits the expression of endogenous debranching enzyme genes. As a result, these cells exhibit a reduced debranching enzyme activity leading to the formation of a modified starch. By using this anti-sense technique it is possible to produce plants in which the expression of endogenous debranching enzyme genes is inhibited in various degrees in a range of 0% to 100%, thereby allowing the production of plants synthesizing amylopectin starch in modified branching degrees. This represents an advantage vis-à-vis conventional breeding and mutagenesis techniques which involve a considerable amount of time and money to provide such a variety. Highly branched amylopectin has a particularly large surface and is therefore specifically suitable as copolymer. A high branching degree furthermore results in an improved solubility in water of the amylopectin. This property is very advantageous for certain technical applications. Particularly suitable for the production of modified amylopectin using the DNA molecules of the invention which code for debranching enzymes is the potato. However, the use of the invention is not limited to this plant species.
The modified starch synthesized in the transgenic plants can be isolated from the plants or plant cells according to conventional methods and can be used for the production of food and industrial products once it is purified. Due to its properties, the starch obtainable from the plant cells and/or plants of this invention is suitable for various industrial applications.
Basically, starch can be subdivided into two major categories, namely hydrolysis products of starch and what are called native starches. Hydrolysis products essentially include glucose obtained by enzymatic or chemical processes as well as glucose building blocks which can be used for further processes, such as fermentation, or further chemical modifications. In this context, it might be of importance that the hydrolysis process can be carried out simply and inexpensively. Currently, it is carried out substantially enzymatically using amyloglucosidase. It is thinkable that costs may be reduced by using lower amounts of enzymes for hydrolysis due to changes in the starch structure, e.g. increase of surface of the grain, improved digestability due to less branching or steric structure, which limits the accessibility for the used enzymes.
The use of what are called native starches, which are used because of their polymer structure, can be subdivided into two large areas:
(a) Use in Foodstuffs
Starch is a classic additive for various foodstuffs, in which it essentially serves the purpose of binding aqueous additives and/or causes an increased viscosity or an increased gel formation. Important characteristic properties are flowing and sorption behavior, swelling and pastification temperature, viscosity and thickening performance, solubility of the starch, transparency and paste structure, heat, shear and acid resistance, tendency to retrogradation, capability of film formation, resistance to freezing/thawing, digestibility as well as the capability of complex formation with e.g. inorganic or organic ions.
(b) Use in Non-foodstuffs
The other major field of application is in the use of starch as an adjuvant in various production processes and/or as an additive in technical products. The major fields of application for the use of starch as an adjuvant are, first of all, the paper and cardboard industries. In this field, the starch is mainly used for retention (holding back solids), for sizing filler and fine particles, as solidifying substance and for dehydration. In addition, the advantageous properties of starch with regard to stiffness, hardness, sound, grip, gloss, smoothness, tear strength as well as the surfaces are utilized.
Within the paper production process, a differentiation can be made between four fields of application, namely surface, coating, mass and spraying.
The requirements on starch with regard to surface treatment are essentially a high degree of brightness, corresponding viscosity, high viscosity stability, good film formation as well as low formation of dust. When used in coating, the solid content, a corresponding viscosity, a high capability to bind as well as a high pigment affinity are important. As an additive to the mass, rapid, uniform, loss-free dispersion, high mechanical stability and complete retention in the paper pulp are of importance. When using the starch in spraying, corresponding content of solids, high viscosity as well as high capability to bind are also of importance.
A major field of application is, for instance, in the adhesive industry, where the fields of application are subdivided into four areas: the use as pure starch glue, the use in starch glues prepared with special chemicals, the use of starch as an additive to synthetic resins and polymer dispersions as well as the use of starches as extenders for synthetic adhesives. 90% of all starch-based adhesives are used in the production of corrugated board, paper sacks, bags, composite materials for paper and aluminum, boxes and wetting glue for envelopes, stamps, etc.
Another possible use as adjuvant and additive is in the production of textiles and textile care products. Within the textile industry, a differentiation can be made between the following four fields of application: the use of starch as a sizing agent, i.e. as an adjuvant for smoothing and strengthening the burring behavior for the protection against tensile forces active in weaving as well as for the increase of wear resistance during weaving, as an agent for textile improvement mainly after quality-deteriorating pretreatments, such as bleaching, dying, etc., as thickener in the production of dye pastes for the prevention of dye diffusion and as an additive for warping agents for sewing yarns.
Furthermore, the starch may be used as an additive in building materials. One example is the production of gypsum plaster boards, in the course of which the starch mixed in the thin plaster pastifies with the water, diffuses at the surface of the gypsum board and thus binds the cardboard to the board. Other fields of application are admixing it to plaster and mineral fibers. In ready-mixed concrete, starch may be used for the deceleration of the sizing process.
Furthermore, the starch is advantageous for the production of means for ground stabilisation used for the temporary protection of ground particles against water in artificial earth shifting. According to state-of-the-art knowledge, combination products consisting of starch and polymer emulsions can be considered to have the same erosion- and incrustation-reducing effect as the products used so far; however, they are considerably less expensive.
Furthermore, the starch may be used in plant protectives for the modification of the specific properties of these preparations. For instance, starches are used for improving the wetting of plant protectives and fertilizers, for the dosed release of the active ingredients, for the conversion of liquid, volatile and/or odorous active ingredients into microcristalline, stable, deformable substances, for mixing incompatible compositions and for the prolongation of the duration of the effect due to a reduced disintegration.
Another important field of application lies in the fields of drugs, medicine and the cosmetics industry. In the pharmaceutical industry, the starch may be used as a binder for tablets or for the dilution of the binder in capsules. Furthermore, starch is suitable as disintegrant for tablets since, upon swallowing, it absorbs fluid and after a short time it swells so much that the active ingredient is released. Medicinal flowance and dusting powders are further fields of application. In the field of cosmetics, the starch may for example be used as a carrier of powder additives, such as scents and salicylic acid. A relatively extensive field of application for the starch is toothpaste.
Also the use of starch as an additive to coal and briquettes is thinkable. By adding starch, coal can be quantitatively agglomerated and/or briquetted in high quality, thus preventing premature disintegration of the briquettes. Barbecue coal contains between 4 and 6% added starch, calorated coal between 0,1 and 0,5%. Furthermore, the starch is suitable as a binding agent since adding it to coal and briquette can considerably reduce the emission of toxic substances.
Furthermore, the starch may be used as a flocculent in the processing of ore and coal slurry.
Another field of application is the use as an additive to process materials in casting. For various casting processes, cores produced from sands mixed with binding agents are needed. Nowadays, the most commonly used binding agent is bentonite with modified starches, mostly swelling starches, mixed in.
The purpose of adding starch is increased flow resistance as well as improved binding strength. Moreover, swelling starches may fulfil more prerequisites for the production process, such as dispersability in cold water, rehydratisability, good mixability in sand and high capability of binding water. In the rubber industry, the starch may be used for improving the technical and optical quality. Reasons for this are improved surface gloss, grip and appearance. For this purpose, the starch is dispersed on the sticky rubberized surfaces of rubber substances before the cold vulcanisation. It may also be used for improving the printability of rubber.
Another field of application for the modified starch is the production of leather substitutes.
In the plastics market, the following fields of application are emerging: the integration of products derived from starch into the processing process (starch is only a filler, there is no direct bond between synthetic polymer and starch) or, alternatively, the integration of products derived from starch into the production of polymers (starch and polymer form a stable bond).
The use of the starch as a pure filler cannot compete with other substances such as talcum. This situation is different when the specific starch properties become effective and the property profile of the end products is thus clearly changed. One example is the use of starch products in the processing of thermoplastic materials, such as polyethylene. In this process, starch and synthetic polymer are combined in a ratio of 1:1 by means of coexpression to form a xe2x80x98master batchxe2x80x99, from which various products are produced by means of common techniques using granulated polyethylene. The integration of starch ill polyethylene films may cause an increased substance permeability in hollow bodies, improved water vapor permeability, improved antistatic behavior, improved anti-block behavior as well as improved printability with aqueous dyes.
Another possibility is the use of the starch in polyurethane foams. Due to the adaptation of starch derivatives as well as due to the optimisation of processing techniques, it is possible to control the reaction between synthetic polymers and the starch""s hydroxy groups. The result are polyurethane films having the following property profiles due to the use of starch: a reduced coefficient of thermal expansion, decreased shrinking behavior, improved pressure/tension behavior, increased water vapor permeability without a change in water acceptance, reduced flammability and cracking density, no drop off of combustible parts, no halides and reduced aging. Disadvantages that presently still exist are reduced resistance to pressure and to impact.
Product development of film is not the only option. Also solid plastics products, such as pots, plates and bowls can be produced having a starch content of more than 50%. Furthermore, the starch/polymer mixtures offer the advantage that they are much easier biodegradable.
Furthermore, due to their extreme capability to bind water, starch graft polymers have gained utmost importance. These are products having a backbone of starch and a side lattice of a synthetic monomer grafted on according to the principle of radical chain mechanism. The starch graft polymers available today are characterized by an improved binding and retaining capability of up to 1000 g water per g starch at a high viscosity. These super absorbers are used mainly in the hygiene field, e.g. in products such as diapers and sheets, as well as in the agricultural sector, e.g. in seed pellets.
What is decisive for the use of the new starch modified by genetic engineering are, on the one hand, structure, water content, protein content, lipid content, fiber content, ashes/phosphate content, amylose/amylopectin ratio, distribution of the relative molar mass, degree of branching, granule size and form as well as crystallisation, and on the other hand, the properties resulting in the following features: flow and sorption behavior, pastification temperature, thickening performance, solubility, paste structure, transparency, heat, shear and acid resistance, tendency to retrogradation, capability of gel formation, resistance to freezing/thawing, capability of complex formation, iodine binding, film formation, adhesive strength, enzyme stability, digestibility and reactivity. Furthermore, viscosity is particularly pointed out.
Furthermore, the modified starch obtainable from the plant cells and/or plants of this invention may be subjected to further chemical modification, which will result in further improvement of the quality for certain of the above-described fields of application or in a new field of application. These chemical modifications are principally known to the person skilled in the art. These are particularly modifications by means of
acid treatment
oxidation
esterification (formation of phosphate, nitrate, sulfate, xanthate, acetate and citrate starches; other organic acids may also be used for the esterification)
formation of starch ethers (starch alkyl ether, O-allyl ether, hydroxylalkyl ether, O-carboxylmethyl ether, N-containing starch ethers, S-containing starch ethers)
formation of branched starches
formation of starch graft polymers.
The invention furthermore relates to the use of the DNA molecules of the invention for the production of plants synthesizing an amylopectin starch having a modified branching degree as compared to that of wild type plants.
A further subject matter of the present invention is the use of the DNA molecules of the invention or of parts of these DNA molecules or of the reverse complements of these molecules for the identification and isolation of homologous molecules coding for proteins having the enzymatic activity of a debranching enzyme, or of fragments of such proteins, from plants or other organisms. For a definition of the term xe2x80x9chomologyxe2x80x9d see above.